1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming apparatus employing an electrophotographic technique and a cleaner-less system.
2. Discussion of the Related Art
In electrophotographic image forming apparatuses, after a toner image formed on a photoconductor serving as an image carrier is transferred onto a transfer member or a recording medium, some small amount of toner particles used to transfer the toner image at times remains on the surface of the photoconductor. Related-art electrophotographic image forming apparatuses have therefore included a cleaning unit for collecting and disposing of such residual toner. However, in response to recent demands for low-cost, compact devices, effective use of resources, etc., manufacturers have proposed and commercialized image forming apparatuses employing a cleaner-less image carrier cleaning system, in which a developing unit serves as a cleaning unit by collecting the residual toner not to dispose of but to reuse.
With the cleaner-less image carrier cleaning system that does not include a cleaning unit for collecting and disposing of the residual toner, any toner remaining on the photoconductor after image transfer is conveyed to the charging unit, and consequently the amount of residual toner on the photoconductor can be greater than that in an image forming apparatus equipped with a regular cleaning unit. Therefore, the toner particles can adhere to and accumulate on the charging unit, which can cause the photoconductor to be charged unevenly and produce defective images as a result.
There are various systems for charging the photoconductor. In particular, a roller charge system in which a conductive charge roller contacts the photoconductor for charging the photoconductor is widely known for its minimal environmental impact (ozone non-production), efficient use of space, stable chargeability, etc. However, as the conductive charge roller contacts the photoconductor, the charge roller can easily pick up residual toner remaining on the photoconductor and the toner adhering to the charge roller can be firmly fixed to the surface of the charge roller over time from friction with and/or pressure against the photoconductor. Toner firmly fixed to the photoconductor can be a cause of uneven surface resistance and can therefore cause uneven charging to the photoconductor, which is likely to produce a defective image with streaks or bands.
There is a different system available in which a cleaning member is disposed in contact with a charge roller to remove residual toner from the charge roller. However, when toner accumulates on the charge roller, it is likely to abruptly degrade cleaning ability of the cleaning member and further require removal from the cleaning member. For example, to prevent toner adhesion to a charging member, related-art image forming apparatuses with a cleaner-less system include a charge control unit (e.g., a brush) disposed upstream from the charging member in a direction of movement of the photoconductor so as to control charging of any residual toner.
Specifically, one related-art image forming apparatus with a cleaner-less system includes a charge control unit that can charge residual toner on a photoconductor to a regular polarity and cause the charge amount of residual toner to be sufficient for a developing unit to develop an electrostatic latent image on the photoconductor into a visible toner image. These controls can avoid causing the residual toner remaining on the photoconductor after transfer to easily adhere to the charge roller held in contact with the photoconductor and can cause the developing unit to collect the residual toner on the photoconductor which should not be used for development.
Further, a different configuration for related-art image forming apparatus with a cleaner-less system has been proposed. In this configuration, the related-art image forming apparatus further includes a cleaning film disposed on the charge roller to charge toner between the cleaning film and the charge roller by frictionally charging the toner to the same polarity as the charge roller for easily transferring toner from the charge roller onto the photoconductor. How to avoid toner adhesion to the charge roller and how to remove toner adhering to the charge roller are two issues that heavily affect the prospects for extending the service life of the charge roller.
Additionally, there are other related-art image forming apparatuses with cleaner-less systems having a configuration in which a charge roller does not contact a photoconductor so that toner does not adhere to the charge roller.
One such related-art image forming apparatus employs a charge roller disposed facing a photoconductor across a gap and having difference in levels of electrical resistance between the surface of the charge roller and a sub-surface portion of the charge roller near the surface. With this configuration, the charge roller does discharge to the photoconductor at a portion of maximum resistance but does not discharge to the photoconductor at a portion of minimum resistance. Therefore, the electrical characteristics of the charge roller can remain stable over time, which can reduce unevenness in charging.
Other related-art image forming apparatuses include a charge roller disposed nearly in contact with a photoconductor to form a gap slightly greater than a diameter of a toner particle. This related-art image forming apparatus includes a cleaning roller to collect residual toner by electrostatically attracting the residual toner on the surface of the photoconductor to an upstream direction of rotation of the photoconductor. With this configuration, the related-art image forming apparatus can switch the polarity of electric potential difference between the cleaning roller and the photoconductor during a period other than an image forming period, collect residual toner to adhere to the photoconductor, and convey the collected residual toner carried on the photoconductor to the developing roller for collection.
However, even if the charge roller is disposed across a gap without contacting the photoconductor, toner remaining on the photoconductor may still adhere to the charge roller and gradually accumulate thereon. Consequently, depending on various conditions, the charging roller may eventually fail to provide an even charge after printing approximately 1,000 sheets.
Therefore, the present inventors have conducted extensive research to determine that contamination of the charging member with time can be significantly reduced by disposing a charging member facing but spaced away from a photoconductor and applying thereto a voltage including an alternating current component. Contamination of the charging member is reduced because toner adhering to the charging member is vibrated due to the alternating current voltage and thrown back onto the photoconductor so that the charging member cleans itself.
By contrast, a non-contact charging system to which a voltage having an alternating current component is applied reduces contamination of the charge roller to acceptable levels. However, since less toner adheres to the charging member, a smaller amount of residual toner is spread over the charging member. Therefore, in a region where a toner image transfer rate is poor, residual toner adhering to a previous image or pattern is conveyed to the charge roller. As a result, toner charge distribution after passing the charge roller may shift to a weakly charged side regardless of the toner charge distribution extant when supplying the residual toner to the charge roller due to the alternating current voltage applied to the charge roller. The shift of the toner charge distribution to the weakly charged side can reduce an image force between the toner and the photoconductor, and this can enhance toner collection. However, when a supply amount of residual toner to the charge roller is significantly large, some toner particles may be charged to a regular polarity intensively, which can hinder toner collection. If the regularly charged toner particles are not collected, these toner particles may cause residual images.
Further, the electric potential of the photoconductor can be easily uneven depending on location. To prevent such unevenness in electrical charge potential, to some extent a strong electrical field needs to be applied. However, such strong electrical fields can produce a proportional amount of discharge products, which can cause problems such as image deletion or image blur due to filming caused by toner adhering to the photoconductor. However, by applying a voltage including an alternating current component to the charge roller disposed facing the photoconductor across a gap, the stability of the electric potential of the photoconductor can be enhanced substantially.
On the other hand, as described above, toner that passes through a gap formed between the charge roller and the photoconductor is vibrated due to the alternating current voltage and thrown onto the photoconductor, and therefore small amounts of toner can adhere to the charge roller to gradually accumulate thereon. Since it is difficult to return the toner that adhering to the charge roller not contacting the photoconductor to the photoconductor, the toner adhering to the charge roller must be returned to the photoconductor by some other means.